1. Field of the Invention
The subject matter of pending application Ser. No. 08/482,958, filed Jun. 15, 1995, is incorporated herein by reference. The invention relates to sensors for sensing gaseous components in the exhaust stream of combustion systems, and more particularly to multi-functional sensors for combustion control systems for selective and simultaneous sensing of more than one gaseous component in the exhaust stream from a combustion system.
2. The Prior Art
Combustion systems convert potential chemical energy in fuels (e.g. natural gas, other hydrocarbons, or hydrogen) into another form of energy such as heat or kinetic energy. The fuel is mixed with air or oxygen and combusted, whereupon the combustion products and unburned fuel and air are exhausted from the system in a gaseous stream. Well-known combustion systems include a wide variety of devices including boilers, furnaces, and reciprocating engines.
Both the efficiency and performance of a combustion system can be improved by regulation of the combustion process. For example, a precise control of the fuel to air ratio can significantly reduce fuel consumption, while simultaneously reducing toxic emissions in the exhaust stream. Regulation is typically accomplished by a control system that monitors the concentration of combustion products such as nitrogen oxides (NO.sub.X), carbon monoxide (CO), unburned fuel and oxygen in the exhaust stream. The relative concentrations of these exhaust components provide information regarding the operation of the combustion system. Important operating parameters such as the fuel to air ratio may then be adjusted to improve operation of the combustion system. Although the requirements of control systems are specific for each different combustion process, the principles governing the control mechanism are similar.
Several problems exist with regard to the current sensing technology used in combustion control systems. Most existing sensors are capable of monitoring only a single exhaust gas component at a time in a combustion process. This component, typically O.sub.2, is continuously monitored and the concentrations of other components are estimated based on mass balance of thermodynamic equilibrium calculations. However, combustion is a highly nonequilibrium process. Thus, concentrations determined in this manner may be quite inaccurate. Potentiometric sensing of other combustion product gases such as NO.sub.X, SO.sub.X, CO.sub.X, and H.sub.2 S, and unspent fuel gases such as CH.sub.4 and C.sub.3 H.sub.8, have been reported but the sensors have demonstrated limited performance. Control systems could be significantly improved if several gaseous components could be accurately monitored, simultaneously.
Several ex-situ techniques based on optical phenomena are capable of multifunctionality, but suffer from other disadvantages. For example Fourier Transform Infrared (FTIR), and Non-Dispersive Infrared (NDIR) sensors can accurately monitor multiple gas concentrations, but are very large and expensive. Further, these sensors require the installation of gas sampling lines which makes the sensors even more cumbersome, and significantly delays control system response. As a result, these techniques are unsuitable for on-line detection, monitoring, and control of combustion systems with a feed back arrangement. Other ex-situ techniques such as Gas Chromatography (GC), Mass Spectrometry (MS), and most traditional chemical analysis methods are less expensive but are also equally cumbersome and time consuming. As such, these are not easily amenable to applications involving on-line detection, monitoring, and control. For these reasons, ex-situ sensors based on FTIR, NDIR, GC, MS, and traditional chemical methods are impractical for combustion control systems.
A need exists for a simple, los-cost, multi-functional sensor capable of sensing more than one gas component simultaneously. The sensor should be reliable and accurate, and be able to withstand the harsh environment of the exhaust stream from an operating combustion system. It has further been known that in lean burning conditions, such as in an internal combustion engine, oxygen concentration in the exhaust gases can be approximately 10-15%. This oxygen concentration is 3-4 orders of magnitude higher than the concentration of nitrogen oxides in the same exhaust gases. Oxygen concentrations of such high magnitudes can affect the operation of sensors configured for the detection of nitrogen oxides.
Accordingly, it is desirable to provide a sensor for the detection of oxides of nitrogen, while operating in an environment of high oxygen concentration. The subject invention relates to such a multi-functional sensor.
This and other objects of the invention will become apparent in view of the present specification, claims and drawings.